Worm Breeder's Gazette 11(1): 63

These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.

Analysis of Egg-laying Muscle Function by Laser Ablation

Michael J. Stern and Bob Horvitz

Figure 1

The hermaphrodite sex muscles, comprising two types of vulval (vm1 & 
vm2) and two types of uterine muscles (um1 & um2), are essential for 
egg laying.  Each of the four SM.x myoblasts give rise to one set of 
muscle cells comprising one of each type of muscle cell.  We have 
determined which of these cells are required for egg laying.
[See Figure 1]
Specific cells were killed by laser ablation in mid-to-late L3 
hermaphrodite larvae and the kills were confirmed 1-4 hours afterwards 
by Nomarski microscopy.  The vulval muscles were visualized in young 
adults by polarized light microscopy to determine if specific ablated 
cells were being replaced by others.  The animals were then scored for 
their abilities to lay eggs.  In general, ablation of specific cells 
resulted in the absence of the corresponding muscle(s), although 
occasionally ablated cells appeared to be replaced.  However, animals 
in which replacement appeared to occur and those in which it did not 
behaved identically in their abilities to lay eggs.  From this result 
we conclude that although the muscle attachments can sometimes 
rearrange, their functions are not fully interchangeable.
To determine whether all four sets are required for egg laying, 
various combinations of the SM.x myoblasts were ablated.  Animals 
missing one set are essentially wild-type, while those missing three 
or all four sets fail to lay any eggs and form bags of worms.  Animals 
with two sets on the same lateral side, and those with an anterior set 
from one side and a posterior set from the other side lay eggs almost 
as well as unablated animals.  In contrast, animals with only two 
anterior or only two posterior sets bloat with eggs.  Thus, it appears 
that some cells from at least one anterior and one posterior set are 
required for effective egg laying.
We also ablated individual types of muscles in all of the sets, 
demonstrating that the vulval muscles are necessary and sufficient for 
normal egg laying and that the vm2's play a more important role than 
the vm1's.  Animals missing all eight uterine muscles lay eggs 
apparently normally, while those missing all eight vulval muscles fail 
to lay any eggs.  Ablation of the four vm2's results in egg-laying 
defective animals that form bags of worms, while ablation of the four 
vm1's results in animals that lay eggs almost as well as unablated 
animals.  The vm2's are the only egg laying-muscles that receive 
direct innervation from the HSN neurons that drive egg laying.  While 
it is possible that the defect caused by ablation of the vm2's is due 
to the inability for the remaining muscles to be stimulated to 
contract, the majority of animals missing the HSN's eventually lay 
their eggs and do not form bags of worms.  Thus, the role of the vm2's 
in opening the uterine/vulval connection is probably also important in 
egg laying even in the absence of innervation by the HSN's.

Figure 1